Gerhard Sauer, G Sauer Ltd, asked why none of the stations had been tunnelled. The answer was that cover at the stations was less than 20m and a box excavation was the most economical solution. John King, Pemberwell, asked what the speaker considered to be the main differences between US and UK tunnellers. Biggart believed there was little difference in terms of rock tunnels but, for soft ground tunnelling, the UK was more advanced. The US also seemed reluctant to embrace EPBMs and PCC linings. The US labour force worked for less money than their UK counterparts and, although there was a growing tendency to claim for personal injury (particularly at the end of the job), there was a diverse labour force often of ethnic minorities, and no apparent sexual discrimination.
John Anderson, consultant, asked who set up partnering and at what stage of the contract; if the contractor had the facility under the contract to claim for unexpected ground conditions; and if the client certified the type of ground to be expected. The contract ‘encouraged’ partnering and this was supported by the client, contract manager, the engineering designer and, eventually, the contractor, who requested partnering be used. The partnering sessions employed a professional facilitator and the joint objectives were clearly set out and signed up to by all the partners. A GDSR geotechnical design summary report was prepared for the benefit of tenderers but, in the event, the contractor had been awarded costs for changed ground behaviour.
Allan Auld, consultant, asked if an even hydrostatic pressure was evident. Biggart was not sure, but the loading on the tunnels showed that they were positioned too close together. Peter Chapman asked why the contractor used ribs everywhere, including the good rock, and where the membrane used in the tunnels was. The contractor used ribs throughout, with an in-situ concrete invert. The geotextile and membrane was used only for the M&E areas and where gas was suspected.
Denis Lawrenson, retired, questioned the classification of the rock before tunnelling started. In his experience, the contractor did not use the information but installed the support to suit the rock as it was excavated. Why not base the class of rock on the actual support used? Biggart agreed that the Q system was an inexact science. The Q classes were established from the initial site investigation carried out on behalf of the client and the contractor had not used the system before and was happy to trust the client’s results. There had been no problem in practice. There was a danger in employing the actual support used as a classification because it would be difficult to stop the contractor from over-supporting.
Dave Wallis, Halcrow, asked: what type of contract was used and who designed temporary and permanent works? What was the type of payment? Did partnering affect the initial support system? Biggart explained that the temporary works were the contractor’s design, the permanent works were all put through to the engineering manager who, under state law, had to be a registered Californian engineer. All changes to the permanent works had to be approved by Mott MacDonald. Payment was lump sum with a small bill of quantities to provide basic rates. Partnering was always on a non-contractual basis and, although there was a 50/50 split on value engineering, there was little opportunity for big value savings; most were less than $100 000.
Neville Harrison, Mott MacDonald, enquired if the TBM choice was the contractor’s. The answer was yes, and he designed the conversion. The grippers were a problem as in the softer ground they slipped and damaged the ribs, which they were supposed to staddle. There was minor input from Mott MacDonald as it had recommended hydraulic drills for probing and the built-in fire precautions.
Mike Macdonnell, Balfour Beattie, asked if Motts influenced the safety plan, and if it was encouraged to cut time costs and programme? Biggart explained that Motts involved itself in safety matters through inspectors on site. There were, unfortunately, some poor safety statistics: three fatalities occurred and accidents requiring treatment were twice the national average. Lost time, however, was only 50% of the average. Motts was paid a fixed fee plus a fee based on performance which was reviewed three times a year by the client. Liquidated damages were imposed on the contractor and, although, some were paid it was not a large amount.
The chairman asked if the speaker had any reservations on the performance of the membrane. Biggart said the membrane was not air tested and in practice had to have a lot of patching, which inevitably led to some leakage and the performance could have been better.
Dick Watts, retired, asked why the tunnels were driven downhill and how the contractor coped with the water. He also wondered why he had erected arch ribs throughout the drives even in the good rock. Biggart explained that the tunnels were driven downhill to reduce the civil work at the Hollywood Boulevard end of the job. Water was pumped from the face without any major problem. Continuous arch support was put in as the geology was so changeable and, as the ribs were bolted together with spacer tubes, it was considered impractical to attempt hit and miss installation.
